Many different medical applications require that patients carry medical sensors on a daily basis. An example of such medical sensors is body temperature sensors, which can either be based on invasive body temperature sensors (arterial line catheters, esophageal/rectal probes, etc.) or non-invasive sensors which are attached to the surface of the subject being monitored.
Experience shows that one of the most importance factors for the patients carrying such non-invasive medical sensors is that they are flexible & stretchable as needed for both high-quality reliable attachment to the body and for ensuring high measurement accuracy and reliability with respect to measurement artifacts. This is definitely the case in case of temperature sensors as they require well-defined stable thermal contact between the skin and the sensor for proper operation. Typically, sensor curvature radius of a few cm (exact curvature is dependent on patient-specific geometry of the sensor placement location) needs to be achievable in case of the temperature sensor that is normally placed on the forehead. Even smaller curvatures of sub-cm scale might be needed when the sensor has to be placed at other locations on the body. In most cases, medical sensors need to be placed either on an ellipsoid-like object or in an ellipsoid-like depression. Therefore, it is not sufficient for the sensors to be able to bend in one direction; they also need to be stretchable.
The use of industry standard manufacturing processes is essential for achieving high yield, high reliability and low manufacturing cost of products. That is especially important in the considered case of consumable medical sensors, where both low cost and high reliability have high priority. Unfortunately, neither standard printed circuit board (PCB) materials nor standard flex-foil materials (e.g. polyimide film) satisfy the requirement of stretchability: PCB substrates are rigid (i.e. neither stretchable nor flexible), and flex-foil substrates are flexible but not stretchable. That makes them ill-suited for the considered class of body-worn anatomically conformal sensors.
The use of alternative substrates (e.g. textiles or rubber sheets) can also be in principle considered, but the corresponding manufacturing processed cannot yet compete with the PCB and flex-foil processed in terms of yield, product reliability and cost. Therefore, it is very much preferred to use the industry-standard PCB or flex-foil (e.g. polyimide) substrates.